1. Field of the Invention
The present invention relates to laminated materials used to make containers, for example, liquid containers, wherein a paperboard base material is laminated with thermoplastic resins, such as polyvinyl alcohol and polyethylene, so that the used laminated containers are easy to recycle.
2. Prior Art
Conventionally, paper-made packaging containers called "pack" containers have been used for vending milk, juice, and other liquid foods. Laminated materials that use paper as the major material (packaging web) have been employed for making ordinary packaging for food that does not require long-term storage (for example, gable-top, sealed paper containers). As shown in FIG. 3, on one side of an ordinary supporting body of a paperboard base material layer 1, forming the outer surface of the container where printed information is provided, a thin thermoplastic synthetic resin layer 3 comprising polyethylene is directly laminated thereto, while on the other side of the paperboard base material layer 1, forming the inner surface of the container, a laminated material made of one or two laminated polyethylene layers with a specified thickness is applied as a liquid-resistant layer 4.
For containers requiring long-term storage after the sterile introduction of the liquid food (ordinarily in cubic form), as shown in FIG. 4, on one side of the supporting body of a paperboard base material layer 1, forming the outer surface of the container, a thin polyethylene layer 3 is directly laminated thereto, while on the other side forming the inner surface of the container, an aluminum foil layer 5 is formed over an adhesive resin layer 6 (ordinarily polyethylene). In turn, over the aluminum foil layer is laminated a material that is made of one or two laminated polyethylene layers to form a liquid-resistant layer 4. Such laminated materials for making packaging containers B1 and B2 are supplied in the form of a long roll with the necessary printed information and folding lines, to a packaging machine for making containers that are then filled with liquid foods.
In recent years, for the purpose of preventing loss of aroma of liquid foods introduced into a packaging container, due to adsorption of aroma by the inner surface of the container, and maintaining the rich aroma of food, laminated materials for packaging, for example, as disclosed by the present inventors in Japanese Patent Publication No. 139334/1990, are provided with a thin film of a silicon compound, such as silicon oxide or silicon nitride, on the innermost surface that contacts the liquid food, over an aluminum or other metal foil layer. A laminated material for packaging provided with an oil-proof function and the ability to withstand microwave heating of its contents without the occurrence of cracks in the sealed layers, and making blank formation possible, has been disclosed, for example, in Japanese Patent Laid-Open Publication No. 139339/1989, in which a sealed layer is provided in the form of an oil-proof paper layer on a paperboard base material layer.
A recent trend has been to recycle pulp and polyethylene and other components that are regenerated from the laminated materials of the containers.
The recycling process of used, empty packaging containers ordinarily starts by washing and shredding the containers with a shredder. The shredded material is then placed into a repulper containing hot water for separating the pulp from other components. The agitator in the repulper is operated so as to separate the pulp in the paperboard base material layer 1 from the polyethylene or from the polyethylene containing aluminum foil or other components. In this procedure, the shredded laminated materials are agitated in hot water in the repulper and pulp fibers in the paperboard base material layer are loosened and dispersed in the polyethylene. The loosened and dispersed pulp fibers are separated from the polyethylene and other components by filtering the mixture through a screen, which is linked to the repulper. The loosened and finely chopped fibers and polyethylene and other components remaining on the filter can then be removed.
As explained above, the shredded laminated materials are agitated in hot water and the pulp in the paperboard base material layer 1 is separated from the thermoplastic synthetic resin (3, 4 or 6), which is ordinarily polyethylene, that is laminated on both sides of the paperboard base material layer 1. In this operation, hot water permeates into the paperboard base material layer 1 to loosen and separate the portion thereof adhering to the thermoplastic resin. However, this loosening step requires time and energy. In addition, after hot water has penetrated into the paperboard base material layer 1, the thermoplastic synthetic resin layers (3, 4 or 6) that are laminated over the paperboard base material layer 1 contains entangled pulp fibers, which requires more time for the separation of pulp fiber and thermoplastic synthetic resin; wherein the separated thermoplastic synthetic resin that has been in contact with the paperboard base material layer 1 contains at least about 5% of residual pulp fibers.